A conventional object detecting apparatus mounted on a vehicle uses a laser light, for instance, to detect an object such as a preceding vehicle or a distance to such an object. This detecting apparatus intermittently drives a laser diode to radiate the laser light towards the forward area of the vehicle, and detects the light reflected from the forward obstacle by a photo sensor. The detecting apparatus measures the distance to the forward obstacle based on a time difference between a light radiation time and a light receiving time.
Specifically, as disclosed in JP 2002-031685A, the detecting apparatus comprises a light radiation unit for radiating a laser light, a polygon mirror and a light receiver unit for receiving a reflected light. The polygon mirror is shaped in a frustum of a hexagonal pyramid and rotatable as a scanning mirror. According to this construction, the polygon mirror reflects the laser light radiated from the light radiation unit and directs it to the forward area of the vehicle.
As the polygon mirror is rotated and the laser light from the light radiation unit is directed to each side surface of the polygon mirror, so that the angle of reflection of the laser light at the polygon mirror is adjusted to scan a predetermined range of the forward area of the vehicle by the laser light. The receiver unit includes a Fresnel lens and a light receiving element to receive the laser light reflected from the forward object and measure the distance thereto.
Various component parts of the apparatus including the above parts are accommodated within a closed case so that a scanning mechanism, optical devices and electronic circuits are protected from frosting of water or foreign matters such as dust.
For passing the laser light through the case, the case is formed to have a light radiating window for radiating the laser light outward to the object and a light receiving window for receiving the laser light reflected by the object. Those windows are made of light -transmitting material, and fit in respective openings provided in the case.
The light radiating window is made of material having high repellency such as a glass. Therefore, as shown in FIGS. 5A and 5B, water droplets become spherical on the surface of the window. The water droplets operate as lenses and the laser light reflected by the polygon mirror is undesirably scattered when radiated to the case outside.
As shown in FIG. 6, the object detecting range of the apparatus is widened under the wet condition, in which the water droplets remain on the outside surface of the light radiating window, than under the dry condition. As a result, the apparatus is likely to erroneously detect an object existing forward of a vehicle as a wider object than it is.